New PoissonBinomial distribution & countreg support & minor improvements

[zeileis]

Alex @alexpghayes, I have accumulated various additions in the devel branch that are ready for the main branch IMO.

If you have the time to look at these: great! If not: no problem. I can either merge myself or wait a little bit longer
because I want to do further additions in the next weeks.

The biggest addition is a new PoissonBinomial distribution, see the Wikipedia for some background. It's a generalization of the binomial distribution where there are still n independent Bernoulli trials but these can have different probabilities.

I have implemented the distribution based on the PoissonBinomial package. However, I have also added fallback computations based on a normal approximation in case the PoissonBinomial package is not available.

In addition to the new distribution I have made various smaller improvements and documentation fix-ups. This includes a distinction in some prodist() methods between the old pscl package and the newer countreg package which we plan to release to CRAN soon (leveraging distributions3).

[alexpghayes]

Looks good to me! Merge as you see fit!

[zeileis]

Thanks, Alex @alexpghayes! I also asked Moritz @mnlang for feedback and now merged it into main.